This invention generally relates to radio frequency (RF)/microwave amplifiers, and in particular, a system and method for automatically improving the impedance match between a load and the output impedance of an output power stage.
When an RF power amplifier is designed, the load variations are taken into account when determining the quiescent biasing condition, or xe2x80x9cQxe2x80x9d point, for the device. Depending on how poor the match can be with a given load, the resultant xe2x80x9cQxe2x80x9d point ends up being compromised. When the condition exists that the magnitude of the load is less than the optimal or characteristic impedance expected by the amplifier, power transfer is no longer optimum.
To achieve the same output power at the load, in the mismatched condition, much more power is dissipated across the output stage. This increase in the power dissipated causes excess heat to be generated in the amplifier which results in an elevated device junction temperature. The elevated junction temperature has an exponential relationship with reliability. In addition to reliability degradation, more power must be supplied to the amplifier than would be required for a properly matched condition. For portable applications which rely heavily on battery capacity, this can drastically increase the time required between recharges.
FIG. 1 illustrates a block diagram of a prior art amplifier output circuit 100. The amplifier output circuit 100 consists of a field effect transistor (FET) 102 having a grounded source (S), and a high RF impedance bias network 104 which directs the power to the output stage. The inherent output impedance for power device 102 is usually much less than the impedance of the load 108. Therefore, for optimum performance, the impedance seen to the left of aaxe2x80x2 is transformed to substantially match the impedance at the load 108. Typically, an impedance matching network 106 is employed to multiply up the output impedance of the transistor 102.
The actual impedance of the transistor 102 is calculated by dividing the voltage drop across the transistor by the current flowing from drain (D) to source (S) at the quiescent bias point with no RF input. This would equate basically to the supply voltage divided by the supplied current to the stage in question. This is true for class A operation.
If the load impedance were to change for some reason, a mismatched condition would exist. This would decrease the efficiency of the system since the criteria for optimum power transfer has been violated. The following discusses the effects of the mismatched conditions when the load impedance increases and decreases.
If the load impedance increases, then the transistor""s output impedance seen at aaxe2x80x2 will be too low for a matched condition. The ac current flowing through the transistor 102, about the xe2x80x9cQxe2x80x9d point current will decrease. The ac voltage, seen at the output, will increase across the load constant. This can only occur until the device starts to approach rail to rail operation. At this point, distortion begins to evidence itself and even though the power may start to approach the desired output power level, the spectral density of this power may become very undesirable since the energy is no longer confined to the desired signal but to intermodulation products as well.
If the load impedance decreases, then the transistor""s output impedance seen at aaxe2x80x2 will be too high for a matched condition. The ac current flowing through the transistor 102, about the xe2x80x9cQxe2x80x9d point current, will increase. The ac voltage, seen at the output, wants to decrease. This can continue only until the device starts to saturate in its ability to provide increased current. At this point, distortion begins to evidence itself and, even though the power may start to approach the desired output power, the spectral density of this power may become very undesirable since the energy is no longer confined to the desired signal but to intermodulation products as well. In addition, there will be an increased voltage drop across as well as current through the output transistor 102. This will cause an increase in junction temperature which will lead to increased stress on the transistor 102 as well as performance degradation.
Accordingly, there is a need to mitigate some of the problems stated above.
One aspect of the invention is a method to compensate an amplifier circuit for changes in a load impedance in order improve the performance of the amplifier. More specifically, if the load impedance increases, then the amplifier is reconfigured to produce an output impedance seen at aaxe2x80x2 that likewise increased. One way of reconfiguring the amplifier for a load impedance increase is to increase the drain (FET) or collector (bipolar) voltage to the device. The increase in the drain (FET) or collector (bipolar) voltage to the device increases the rail to rail operation capability of the device. This would allow more dynamic range for the system performance. Assuming the drain (FET) or collector (bipolar) current is substantially constant, the impedance seen at aaxe2x80x2 will increase and be multiplied up, by the impedance matching network, to the impedance desired by the load resulting in more optimum power transfer.
Similarly, if the load impedance decreases, then the amplifier is reconfigured to produce an output impedance seen at aaxe2x80x2 that likewise decreases. One way of reconfiguring the amplifier for a load impedance decrease is to decrease the drain (FET) or collector (bipolar) voltage to the device. Assuming the drain (FET) or collector (bipolar) current is substantially constant, the impedance seen at aaxe2x80x2 will decrease and be multiplied up, by the impedance matching network, to an impedance closer to that desired by the load for a more optimum power transfer.
In addition to changing the supply voltage to the amplifier for tuning its output circuit with the load, the drive input and the gate voltage (FET) or base current (bipolar) to the amplifier can also be changed to improve the impedance match between the output of the amplifier and the load. Specifically, if the load impedance increases, then the input drive to the amplifier is increased, and if the load impedance decreases, the input drive to the amplifier is decreased. Also, if the load impedance increases, the gate voltage (FET) or base current (bipolar) to the amplifier is changed to decrease the conduction current through the device, and if the load impedance decreases, the gate voltage (FET) or base current (bipolar) to the amplifier is changed to increase the conducting current through the device.
Other aspects of the invention includes an amplifier comprising an output amplification stage, a directional coupler for generating signals indicative of forward and reverse powers between the output amplification stage and a load, and a controller for determining an impedance match between the output amplification stage and the load from the signals indicative of forward and reverse powers. The controller is capable of changing an input drive to the output amplification stage in response to a change in an impedance of the load to improve the impedance match between the output amplification stage and the load. The controller can also change the drain and/or gate voltages (FET), or collector voltage and/or base current (bipolar) to the amplifier along with the input drive, individually or in any combination, to improve the impedance match between the output amplification stage and the load.
An additional aspect of the invention includes a method of tuning an amplifier, comprising the steps of changing an input drive to the amplifier along with the drain and/or gate voltages (FET), or collector voltage and/or base current (bipolar) to the amplifier, individually or in any combination, to improve the performance of the amplifier. The tuning of the amplifier by changing the variables listed above may be in response to a change in the load impedance. In such a case, it may be desirable to determine the impedance match between the output of the amplifier and the load.
Still, yet another aspect of the invention includes an amplifier comprising a plurality of cascaded amplification stages including an output amplification stage, a directional coupler for generating signals indicative of forward and reverse powers between the output amplification stage and a load, and a controller for determining an impedance match between the output amplification stage and the load from signals indicative of forward and reverse powers, and for changing the operating conditions of respective amplification stages to improve the impedance match between the output amplification stage and the load. The operating conditions of the respective amplification stages changed include the drain (or collector) voltage, input drive and/or the gate voltage (or base current). A directional coupler may be included between each of the stages of an amplifier, or between some of the stages.
Other aspects of the invention will become apparent from the detailed discussion of the invention as provided below.